1. Field of the Invention
The present invention relates to an impedance adjusting device and, particularly, to a power impedance adjusting device that adjusts impedance of a switching power supply for an electronic device to play audio signals without distortions.
2. Description of the Related Art
In an ideal condition, an impedance of a capacitor decreases as an operating frequency of the capacitor increases. But actually, the capacitor has a capacitance and a parasitic inductance. Therefore, the impedance of the capacitor includes both the capacitive impedance and inductive impedance. When the input frequency equals an impedance related with frequency of the capacitor, the capacitor has the smallest impedance. Namely, when the input frequency is larger than the impedance related with frequency, the impedance of the capacitor increases as the input frequency increases because of the parasitic inductance, and when the input frequency is smaller than the impedance related with frequency, the impedance of the capacitor decreases as the input frequency increases because of the capacitance.
With reference to FIG. 9 and FIG. 10, when the input frequency increases gradually, the impedance of the capacitor decreases first and then increases. Each capacitor has a particular impedance related with frequency, and when the input frequency equals the impedance related with frequency of the capacitor, the capacitor has the smallest impedance.
With reference to FIG. 11, an electronic device 60 receives electric power from a switching power supply 50, and can output audio signals whose frequencies are in an audible range, such as between 20 Hz and 20000 Hz. The electronic device 60 has a digital to analog converter 61 to output the audio signals and an amplifier 62 to amplify the audio signals. The digital to analog converter 60 is electrically connected to the amplifier 62 to output the audio signals to the amplifier 62, and is electrically connected to the switching power supply 50 to receive electric power from the switching power supply 50. The amplifier 62 amplifies the audio signals, and outputs the amplified audio signals to an external audio device 70 for playing the amplified audio signals at larger volume. When the external audio device 70 plays the amplified audio signals, a current will go through the switching power supply 50, the amplifier 62, and the external audio device 70. Therefore, an impedance of a capacitor C mounted in the switching power supply 50 connected to the electronic device 60 is influenced by frequencies of the audio signals. Since an output impedance of the switching power supply 50 corresponds to the impedance of the capacitor C and a gain of the amplifier corresponds to the impedance of the output capacitor C, the gain of the amplifier is influenced according to variation of the impedance of the capacitor C. The capacitor C acts as an AC filter in a DC mode and an impedance in an audio signal mode. The capacitor C, made by the roll aluminum foil type, will contain an inductance effect to the audio signal mode. The impedance of the capacitor C to the audio frequency will be
  R  =            1              2        ⁢        π        ⁢                                  ⁢        fc              +          2      ⁢                          ⁢      π      ⁢                          ⁢      fL      as FIG. 9. The related audio signal output voltage to the external audio device will be as FIG. 10. The frequency of the audio signal output voltage is not a flat response, that can not perform a full spectrum of 20 Hz to 20 KHz of the audio signal and needs to be solved.